Method for Fabricating Isolation Structures for Flash Memory Semiconductor Devices

ABSTRACT

A method for fabricating integrated circuit devices, e.g., Flash memory devices, embedded Flash memory devices. The method includes providing a semiconductor substrate, e.g., silicon, silicon on insulator, epitaxial silicon. In a specific embodiment, the semiconductor substrate has a peripheral region and a cell region. The method includes forming a first dielectric layer (e.g., silicon dioxide) having a first thickness overlying a cell region and a second dielectric layer (e.g., silicon dixode) having a second thickness overlying the peripheral region. In a specific embodiment, the cell region is for Flash memory devices and/or other like structures. The method forms a pad oxide layer overlying the first dielectric layer and forms a nitride layer overlying the pad oxide layer. The method includes patterning at least the nitride layer to expose a first trench region in the peripheral region and to expose a second trench region in the cell region, while a portion of the first dielectric layer having the first thickness in the cell region is maintained. The method includes forming a first trench structure having a first depth in the first trench region, while the portion of the first dielectric layer having the first thickness in the cell region protects the second trench region. The method includes removing the portion of the first dielectric layer to expose the second trench region. In a specific embodiment, the method includes subjecting the first trench region, including the first trench structure, and the second trench region with an etching process to continue to form the first trench structure from the first depth to a second depth and to form a second trench structure having a third depth within the second trench region. In the third depth is less than the second depth.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to Chinese Application No.200510111386.4; filed on Dec. 5, 2005; commonly assigned, and of whichis hereby incorporated by reference for all purposes.

BACKGROUND OF THE INVENTION

The present invention is directed to integrated circuits and theirprocessing for the manufacture of semiconductor devices. Moreparticularly, the invention provides a method and structure formanufacturing isolation structures for memory devices. Merely by way ofexample, the invention has been applied to the manufacture of Flashmemory devices. But it would be recognized that the invention has a muchbroader range of applicability.

Integrated circuits have evolved from a handful of interconnecteddevices fabricated on a single chip of silicon to millions of devices.Conventional integrated circuits provide performance and complexity farbeyond what was originally imagined. In order to achieve improvements incomplexity and circuit density (i.e., the number of devices capable ofbeing packed onto a given chip area), the size of the smallest devicefeature, also known as the device “geometry”, has become smaller witheach generation of integrated circuits.

Increasing circuit density has not only improved the complexity andperformance of integrated circuits but has also provided lower costparts to the consumer. An integrated circuit or chip fabricationfacility can cost hundreds of millions, or even billions, of U.S.dollars. Each fabrication facility will have a certain throughput ofwafers, and each wafer will have a certain number of integrated circuitson it. Therefore, by making the individual devices of an integratedcircuit smaller, more devices may be fabricated on each wafer, thusincreasing the output of the fabrication facility. Making devicessmaller is very challenging, as each process used in integratedfabrication has a limit. That is to say, a given process typically onlyworks down to a certain feature size, and then either the process or thedevice layout needs to be changed. Additionally, as devices requirefaster and faster designs, process limitations exist with certainconventional processes and materials.

An example of such a process is the manufacture of isolation structuresbetween the MOS devices. Such device has traditionally became smallerand smaller and produced faster switching speeds. Although there havebeen significant improvements, such device designs still have manylimitations. As merely an example, these designs must become smaller andsmaller but still provide clear signals for switching, which become moredifficult as the device becomes smaller. Additionally, these designs areoften difficult to manufacture and generally require complexmanufacturing processes and structures. These and other limitations willbe described in further detail throughout the present specification andmore particularly below.

From the above, it is seen that an improved technique for processingsemiconductor devices is desired.

BRIEF SUMMARY OF THE INVENTION

According to the present invention, techniques for processing integratedcircuits for the manufacture of semiconductor devices are provided. Moreparticularly, the invention provides a method and structure formanufacturing isolation structures for memory devices. Merely by way ofexample, the invention has been applied to the manufacture of Flashmemory devices. But it would be recognized that the invention has a muchbroader range of applicability.

In a specific embodiment, the present invention provides a method forfabricating integrated circuit devices, e.g., Flash memory devices,embedded Flash memory devices. The method includes providing asemiconductor substrate, e.g., silicon, silicon on insulator, epitaxialsilicon. In a specific embodiment, the semiconductor substrate has aperipheral region and a cell region. The method includes forming a firstdielectric layer (e.g., silicon dioxide) having a first thicknessoverlying a cell region and a second dielectric layer (e.g., silicondioxide) having a second thickness overlying the peripheral region. In aspecific embodiment, the cell region is for Flash memory devices and/orother like structures. The method forms a pad oxide layer overlying thefirst dielectric layer and forms a nitride layer overlying the pad oxidelayer. The method includes patterning at least the nitride layer toexpose a first trench region in the peripheral region and to expose asecond trench region in the cell region, while a portion of the firstdielectric layer having the first thickness in the cell region ismaintained. The method includes forming a first trench structure havinga first depth in the first trench region, while the portion of the firstdielectric layer having the first thickness in the cell region protectsthe second trench region. The method includes removing the portion ofthe first dielectric layer to expose the second trench region. In aspecific embodiment, the method includes subjecting the first trenchregion, including the first trench structure, and the second trenchregion with an etching process to continue to form the first trenchstructure from the first depth to a second depth and to form a secondtrench structure having a third depth within the second trench region.In the third depth is less than the second depth.

Many benefits are achieved by way of the present invention overconventional techniques. For example, the present technique provides aneasy to use process that relies upon conventional technology. In someembodiments, the method provides higher device yields in dies per wafer.Additionally, the method provides a process that is compatible withconventional process technology without substantial modifications toconventional equipment and processes. Preferably, the invention providesfor an improved process integration for design rules of 90 nanometersand less for high density Flash devices. In a preferred embodiment, weare able to form dual STI trench depth with no additional reticle ormasking steps required for a Flash memory process. This method will gainadvantages of having shallower trench depth in cell array at minimumcost. Depending upon the embodiment, one or more of these benefits maybe achieved. These and other benefits will be described in morethroughout the present specification and more particularly below.

Various additional objects, features and advantages of the presentinvention can be more fully appreciated with reference to the detaileddescription and accompanying drawings that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 9 are simplified diagrams of a method for fabricating aFlash memory device according to an embodiment of the present invention.

FIG. 10 is a simplified top and cross-sectional views of a Flash memorydevice according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, techniques for processing integratedcircuits for the manufacture of semiconductor devices are provided. Moreparticularly, the invention provides a method and structure formanufacturing isolation structures for memory devices. Merely by way ofexample, the invention has been applied to the manufacture of Flashmemory devices. But it would be recognized that the invention has a muchbroader range of applicability.

Self aligned source (SAS) is widely used in ETOX type Flash Memoryprocesses in order to reduce the cell size along bit-line direction. Atthe same time, it is often required to have low source line resistancein order to minimize the unwanted voltage drops along the source line.Isolation pitch, trench depth and source junction profiles are the keyfactors to determine the source line resistance. As process featureshrinks, deeper trench (˜3500 Angstroms) is widely adopted in standardlogic processes in order to meet the stringent isolation requirement.This negatively impact the source line resistance by not onlyeffectively increases the total length of source line, but alsointroduces difficulty to dope the sidewall of the trench. It istherefore suggested to implement dual trench scheme, i.e., two differenttrench depth with shallower trench in Flash cell array and deeper trenchin peripheral circuit. Further details of the present invention can befound throughout the present specification and more particularly below.

A method for fabricating a Flash memory according to an embodiment ofthe present invention can be outlines briefly as follows:

1. Provide silicon substrate;

2. Grow a layer of pad oxide (˜100 Angstrons) on entire siliconsubstrate;

3. Deposit a layer of nitride (˜1500 Angstroms);

4. Apply photo resist and do exposure with cell Vt mask;

5. Etch away the nitride opened by cell Vt mask and do Cell Vt implant;

6. Remove the photo resist and strip away the remaining pad oxide in theopened area;

7. Grow a layer of thick sacrificial oxide (˜500 Angstroms) in theopened area, while other area are protected by the nitride;

8. Remove the remaining nitride and pad oxide layer to make the surfaceof the silicon outside the cell array to be at same level as the oxidesurface inside the cell array;

9. Grow a layer of pad oxide (˜100 Angstroms) again followed bydepositing a layer of nitride (˜1500 Angstroms);

10. Apply photo resist (with suitable anti-reflective coating ifnecessary) and provide exposure with AA mask;

11. Perform nitride etch and pad oxide etch, followed by 1st silicontrench etch to certain depth (˜1000 Angstroms);

12. Switch etching chemistry to do oxide etch so as to remove all of theoxide inside the cell array area;

13. Switch etching chemistry again back to silicon trench etch to etchsilicon both inside and outside cell array;

14. Complete the trench etch with proper bottom corner rounding, suchthat the trench depth in peripheral is deeper than cell array, e.g. 3700Angstroms versus 2700 Angstroms;

15. Remove the photo resist and clean the wafer.

16. Form shallow trench isolation (STI) liner oxide;

17. Form isolation material with gap fill in the trenches (and usereverse active mask etch;

18. Perform chemical mechanical polishing to planarize the oxide andnitride regions;

19. Remove the nitride and the pad oxide in both cell and peripheralactive area (It is desired to have the thicker sacrificial oxide in cellarray with faster wet removal rate than the thin pad oxide in peripheralarea as the former oxide has gone through cell Vt implant);

20. Perform other steps for Flash memory device; and

21. Perform other steps, as desired.

The above sequence of steps is a method according to an embodiment ofthe present invention. As shown, the method provides a method forforming a flash memory device structure using a method for formingtrench isolation structures for improved device integration according toa specific embodiment of the present invention. Other alternatives canalso be provided where steps are added, one or more steps are removed,or one or more steps are provided in a different sequence withoutdeparting from the scope of the claims herein. Further details of thepresent method can be found throughout the present specification andmore particularly below.

FIGS. 1 through 9 are simplified diagrams of a method for fabricating aFlash memory device according to an embodiment of the present invention.These diagrams are merely examples, which should not unduly limit thescope of the claims herein. One of ordinary skill in the art wouldrecognize many variations, modifications, and alternatives. As shown,the present invention provides a method for fabricating a Flash memoryaccording to an embodiment of the present invention. As shown, themethod begins by providing a semiconductor substrate 101, e.g., silicon,silicon on insulator, epitaxial silicon. The substrate has at least acell region 103 and a peripheral region 101. The method includes forminga dielectric layer overlying the cell region and the peripheral region.In a preferred embodiment, the method grows a layer of pad oxide 105(˜100 Angstrons and less, or slightly more) on an entire siliconsubstrate. In a specific embodiment, the method includes forming asilicon nitride layer 107 overlying the pad oxide layer. In a preferredembodiment, the method deposits a layer of silicon nitride at athickness of about 1500 Angstroms or less, or slightly more. As shown,the method forms a masking layer 109, which exposes the cell region. Ina preferred embodiment, the method applies a photo resist layer andprovides exposure with a cell Vt mask. The method removes via etchingthe nitride opened by cell Vt mask and forms the cell Vt implantaccording to a specific embodiment.

Referring now to FIG. 2, the method removes the photo resist and stripsaway the remaining pad oxide in the opened area, as shown. The photoresist is often removed by an ashing process or like process. The padoxide is removed using a selective etching technique that removes thepad oxide to expose a bare silicon surface according to a specificembodiment. The method forms a thick layer of sacrificial materialoverlying the cell region according to a specific embodiment. In apreferred embodiment, the method uses a thermal technique to grow alayer of thick sacrificial oxide (˜500 Angstroms) in the opened area,while other the areas are protected by the nitride layer. That is, theother area includes at least the peripheral region, as shown.

Referring to FIG. 3, the method removes the remaining nitride and padoxide layer and cause the surface of the silicon outside the cell arrayto be at same level 301 as the oxide surface inside the cell array. In aspecific embodiment, the present method of removing the nitride layeruses a dry etching technique that selectively removes the nitride andpad layer. Of course, there can be other variations, modifications, andalternatives.

In a specific embodiment, the method includes forming a pad oxide layerand an overlying nitride layer, which serves as a masking layer.Referring to FIG. 4, the method grows a layer of pad oxide 408 (˜100Angstroms and less, or slightly more) again followed by depositing alayer of nitride 401 (˜1500 Angstroms) according to a specificembodiment. In a specific embodiment, the method applies a photo resist403 (with suitable anti-reflective coating if necessary) and provideexposure with AA mask, as shown. Exposured regions 405 are illustrated.As also shown, region 402 is thicker than region 408 according to apreferred embodiment. Of course, there can be other variations,modifications, and alternatives.

The method includes performing an etching step to pattern the nitridelayer, which now serves as a masking material 503 according to aspecific embodiment, as illustrated by FIG. 5. The method performsnitride etch and pad oxide etch, followed by 1st silicon trench etch toa certain depth. In a specific embodiment, the depth can be about 1000Angstroms and less, or slightly more. As shown, oxide layer 402 ismaintained in the cell region, while trench regions 501 have beenexposed, according to a specific embodiment.

In a specific embodiment, the method removes exposed portions of theremaining oxide layer, as shown in FIG. 6. In a preferred embodiment,the method changes etching chemistry to perform an oxide etch to removeall and/or substantially all of the oxide inside the cell array area. Ofcourse, there can be other variations, modifications, and alternatives.

Referring to FIG. 7, the method performs an etching step to form trenchregions on the exposed areas. That is, the method changes the etchingchemistry again back to silicon trench etch to etch silicon both insideand outside cell array. In a specific embodiment, the method forms firsttrench regions, which have a first depth, second trench regions 706including first 705 and second depths 701, and third trench regions withsecond depths 703. In a specific embodiment, the terms “first” “second”and “third” are not intended to be limiting but merely provided forexplanation.

In a specific embodiment, the method completes the trench etch with aproper bottom corner rounding, such that the trench depth in theperipheral region is deeper than trench depth in the cell array region,e.g. 3700 Angstroms versus 2700 Angstroms. Additionally, the roundedregions have a radius of curvature of about 100 Angstroms and moreaccording to a specific embodiment. In a specific embodiment, the methodremoves the photo resist and performs a cleaning process on the surfacesof the semiconductor substrate.

Referring to FIG. 8, the method forms isolating material in the trenchregions. In a specific embodiment, the method forms a shallow trenchisolation (STI) liner oxide and deposits a gap fill oxide material 801.The method also uses a reverse active mask etch and chemical mechanicalpolishing (CMP) process to planarize the regions of nitride 803 andoxide 807 on the semiconductor substrate according to a specificembodiment. In a specific embodiment, the reverse active mask can beopened on the cell region to improve STI CMP uniformity across the celland peripheral area. The method removes the nitride and the pad oxide inboth cell and peripheral active areas, as shown in FIG. 9. Dependingupon the embodiment, it is desired to have a thicker sacrificial oxidein cell array with a faster wet removal rate than the thin pad oxide inperipheral area as the former oxide has gone through cell Vt implant,which causes impurities to be in the oxide layer. In a preferredembodiment, the faster etch rate can reduce the divot 901 formed at theSTI corner in peripheral area due to longer wet etch time required totake away the thick sacrificial oxide in cell area. Of course, there canbe other variations, modifications, and alternatives.

In a specific embodiment, the method performs steps to form the Flashmemory device. The Flash memory device is often an ETOX type device, butcan be others. As merely an example, FIG. 10 illustrates a top andcross-sectional views of the Flash memory device according to anembodiment of the present invention.

The above sequence of steps is a method according to an embodiment ofthe present invention. As shown, the method provides a method forforming a flash memory device structure using a method for formingtrench isolation structures for improved device integration according toa specific embodiment of the present invention. Other alternatives canalso be provided where steps are added, one or more steps are removed,or one or more steps are provided in a different sequence withoutdeparting from the scope of the claims herein.

1. A method for fabricating integrated circuit devices, the methodcomprising: providing a semiconductor substrate, the semiconductorsubstrate having a peripheral region and a cell region; forming a firstdielectric layer having a first thickness overlying a cell region and asecond dielectric layer having a second thickness overlying theperipheral region; forming a pad oxide layer overlying the firstdielectric layer; forming a nitride layer overlying the pad oxide layer;patterning at least the nitride layer to expose a first trench region inthe peripheral region and to expose a second trench region in the cellregion, while a portion of the first dielectric layer having the firstthickness in the cell region is maintained; forming a first trenchstructure having a first depth in the first trench region, while theportion of the first dielectric layer having the first thickness in thecell region protects the second trench region; removing the portion ofthe first dielectric layer to expose the second trench region; andsubjecting the first trench region, including the first trenchstructure, and the second trench region with an etching process tocontinue to form the first trench structure from the first depth to asecond depth and to form a second trench structure having a third depthwithin the second trench region; whereupon the third depth is less thanthe second depth.
 2. The method of claim 1 wherein the first thicknessis formed by forming an initial pad layer overlying the cell region,oxidizing the initial pad layer to increase a thickness of the initialpad layer.
 3. The method of claim 1 further comprising removing thefirst dielectric layer having the second thickness in the peripheralregion.
 4. The method of claim 1 further comprising forming one or moreFlash memory devices on the cell region.
 5. The method of claim 1wherein the third depth is 2000 to 4000 Angstroms.
 6. The method ofclaim 1 wherein the second depth is 3000 to 5000 Angstroms.
 7. Themethod of claim 1 wherein the nitride layer has a thickness of about1000 to about 2000 Anstroms.
 8. The method of claim 1 wherein the firstthickness is at least 200 Angstroms.
 9. The method of claim 1 whereinthe second thickness is at least 100 Angstroms.
 10. The method of claim1 wherein the second trench structure has a width of about 0.2 micronand less.
 11. The method of claim 1 further comprising filling the firsttrench structure and the second trench structure with a dielectricmaterial.
 12. The method of claim 11 further comprising removing thedielectric material in the second trench structure and implantingimpurities within the second trench structure to form a self alignedsource region.
 13. A flash memory device comprising: a semiconductorsubstrate, the semiconductor substrate having a peripheral region and acell region; a first trench isolation region in the peripheral region,the first trench isolation region having a first depth; and a secondtrench isolation region having a second depth, the second depth beinglarger than the first depth; and a flash memory device formed in thecell region and being isolated by the first trench isolation region.